In recent years, there has been a progress in size reduction and thinning of encoders. While the size of prevalent detecting heads is in a range of 10 mm to 20 mm, optical encoders with detecting heads of a few mm wide in size are also coming out in the market. Because of such a trend of size reduction, it is assumed that even in packaging, size reduction will have the influence over the shape and form of encoders.
For instance, it is assumed that further thinning of conventional packages in which, a metal or ceramics is used, and replacing the conventional packages by packages in which, a packaging technology such as resin molding is used, will be taking place.
As a conventional example of an optical encoder in which, an attention has been paid to the size reduction and slimming, an encoder indicated in patent literature 1 is cited. FIG. 15 shows a cross-sectional view of the encoder of the conventional example.
It is a reflection-type optical encoder which has a light emitting element 42 and photoelectric conversion element arrays 43 and 44 in a detecting head. The light emitting element 42 and the photoelectric conversion element arrays 43 and 44 are covered by a transparent optical member 45, and the transparent optical member 45 is a packaging member as it has been.
In this conventional example, conditions such as the following are added. In other words, when the distance from the light emitting section 42 up to the farthest location of a light receiving surface of the photoelectric conversion element arrays 43 and 44 is let to be D, the distance from the light emitting surface of the light emitting section 42 and the light receiving surface of the photoelectric conversion element arrays 43 and 44 up to the surface of the transparent optical member 45 is let to be G, and the angle at which reflectance of the inner surface of the transparent optical member 45 is 10% is let to be θ, the thickness G is set such that the following expression holds true.G≦D/(2·tan θ)
For facilitating the size reduction by molding the light emitting section 42 which is a light emitting element of the detecting head, and the photoelectric conversion element arrays 43 and 44 with the transparent optical member 45, when the thickness of the transparent optical member 45 which is a protective member is reduced, light which emerges from the light emitting section 42 will be reflected at an inner surface of the transparent optical member 45 then be incident on the photoelectric conversion element arrays 43 and 44. Therefore, an SN ratio of a signal is deteriorated. Therefore, the thickness of the transparent optical member 45 is let to be not less than a fixed value, and the reflectance of the inner surface of the transparent optical member 45 is let to be not more than 10%.    Patent literature 1: Japanese Patent Application Laid-open Publication No. 2005-156549